Elongated tubesheets for hollow fiber type battery cells and method of fabricating the same

ABSTRACT

Several advantages as to ease of cell fabrication, maximum cell size, fiber breakage, tubesheet tightness, resistance to tubesheet deformation in prolonged service, safety, etc., can be realized by using as the tubesheet in a hollow fiber type battery cell one which is elongated in shape, has a substantially smaller diameter than the fiber &#34;bundle&#34; depending from it and in which the fiber ends passing through it are closely packed.

BACKGROUND OF THE INVENTION

Hollow fiber type battery cells--as exemplified by high temperature,sodium/sulfur cells--have heretofore utilized disc-shaped tubesheets.See U.S. Pat. Nos. 3,476,062; 3,672,995; 3,697,480; 3,703,412;3,749,603; 3,765,944; 3,791,868; 3,829,331; 3,917,490; 4,050,915;4,219,613; 4,224,386; 4,296,052; 4,332,868 and 4,403,742 the disclosuresof which are incorporated herein by reference, for all purposes whichlegally may be served thereby.

In the cells disclosed in the latter patents, the fibers terminate andopen upon the "outer" or "upper" face of the tubesheet and depend asclosed-ended lengths from the inner or lower face. The open endscommunicate with an anode compartment and the closed-endedportions--together with intervening wraps of a currentcollecting/distributing aluminum foil--are immersed in the catholyte.The tubesheet periphery is sealingly engaged with at least the anolytecontainer and thus separates the anode and cathode materials; it is alsoelectronically non-conductive. The fiber walls are "permeable" tocations of the anode material (molten sodium, for example), and the(conductive) anode material and the foil are connected by electricallead posts to an external electrical circuit when the cell is inoperation.

Particularly pertinent to the present invention are two patents in thepreceding list, namely, U.S. Pat. Nos. 4,219,613 and 4,296,052. The '613patent is directed to an assembly of a hollow fiber bundle and ahelium-tight tubesheet of graded porosity (the latter resulting inbetter stress distribution on the fibers where they "enter" thetubesheet). The '052 patent concerns a two-step ("bake and broil")method of attaining the graded porosity during "firing" of thetubesheet, and constitutes the nearest known prior art firing method.

Difficulties have been encountered in attempting to scale up cells ofthe type disclosed in the above-listed patents. For example, as aconsequence of the sizes of some of the particles from which thetubesheets employed must be formed and of the low glass transitiontemperatures of the solder glasses which have been found suitable forthe methods of tubesheet fabrication used, the disc-shaped tubesheetsemployed in the prior art cells are too easily deformed, at elevatedcell operation temperatures. Under the combined influences of gravityand the pressure differential resulting from transfer of material fromthe anode compartment to the cathode compartment during discharge,deformation occurs. The extent of deformation in a single dischargehalf-cycle is not large but the incremental deformation is noteffectively reversed during recharging of the cell. Thus, deformationaccumulates during prolonged charge/discharge cycling of the cell. Thisproblem (which is not disclosed or suggested in the prior art) would beexpected to become rapidly more severe as the diameter of the tubesheetis increased. Since as little as about a 10 mil (254 micron) deformationmay cause fiber breakage, it will be appreciated that deformability is areal obstacle to scale up of cells in which the tubesheet is of the disctype.

Another, considerable, obstacle is the difficulty of fabricating larger,leak-free assemblies of hollow fiber bundles and disc-type tubesheets bythe one-step method disclosed in the prior art. That is, when a "ladder"of parallel fiber lengths, wrap-spacing and fiber-spacing foil tapes anda cathode foil are rolled up together around a rotating, horizontalmandrel, the portions of the fibers which are to extend through thetubesheet extend substantially beyond the end of the mandrel. Theseprotruding portions are unsupported and are deflected downwardly as thetubesheet material (a pasty slurry of glass particles in avolatilizeable liquid medium) is deposited on (and between) them. As thedeveloping roll rotates, the sagging fiber portions are lifted, bowedeven more and subjected to a twisting action. The resultant flexingmakes proper placement of more slurry awkward at best and is not veryhelpful to formation of a body in which the slurry is uniformlydistributed around and between the fibers; it is difficult to preparetubesheet/fiber assemblies by this method which do not require some typeof post-firing treatment to render them leak-free. This is particularlyso for larger diameter tubesheets of the prior art type.

It is possible to make useable disc-type tubesheets by other than the"bake and broil" method. That is, the advantages of a graded porosity inthe tubesheet may be dispensed with for the sake of the lower number ofleak paths which results when the tubesheet is more uniformly densified.However, this requires extremely critical, close control of the firingtime and temperature, in order to ensure adequate melting of thetubesheet material and bonding to the fibers, without closing off anexcessive proportion of the fibers at the same time.

Thus, it is apparent that a tubesheet configuration which would avoidthe foregoing problems is highly to be desired.

Another problem with disc-type tubesheets--particularly the more highlydensified versions thereof--is that substantial contraction occurs asthe liquid slurry medium is removed and densification occurs. Thegreatest displacement, by circumferential and radial shrinkage, occursin the peripheral portion of the tubesheet, resulting in "mud-cracking".Thus, as the tubesheet diameter is increased, the cracking becomes soextensive as to render the disc non-functional for its intended purpose.

It will be seen that the most readily apparent solution to thedeformation problem, thickening the tubesheet, would aggravate, atleast, the difficulty of forming the "green" tubesheet/fiber (etc.)assembly. However, if the amount of the tubesheet material could bereduced, i.e., if the spacing between the fibers within the tubesheetcould be considerably reduced, an elongated tubesheet of correspondinglyreduced diameter would result. The tubesheet would be highly resistantto deformation. But another problem is posed. The latter modificationwould require holding the fiber ends together during introduction of theslurry or "squeezing out" some of the slurry; neither expedient workedwhen tried. It would also necessitate spacing the tubesheet further outalong the mandrel axis from the rest of the assembly, to avoidexcessively sharp bending of the outermost fibers, particularly uponscale up to larger diameter assemblies. This would increase the saggingand flexing during rolling, due to the greater leverage and even thoughless slurry was applied.

In addition, the spherical particles included with the ground glass inthe tubesheet slurry (to ensure extrudeability of the pasty slurry andefficient, more uniform particle packing in the green tubesheet) wouldhave to be removed in order to attain close packing of the fiber ends.This in turn would require using a more dilute slurry (to retainextrudeability) and would have the consequence of even greater shrinkageduring drying and firing of the tubesheet. It would also make initialretention of the slurry on the rotating fiber "brush" much moredifficult.

Yet another apparent difficulty with going to an elongate or "plug"tubesheet configuration is that the butt-type seals employed in theprior art for joining the cathode (and anode) cup(s) to the periphery ofthe disc-type tubesheet would not appear to be very practical forrelatively small diameter tubesheets. This would necessitate using anoverlapping, concentric ("sleeve") type of seal and such seals arenotoriously more difficult to form as strain-free bodies.

It will be recognized that a plug tubesheet--if somehowattainable--would have a substantial safety benefit. That is, breakageof a disc-type tubesheet in an active cell can result in immediatecontact between relatively large amounts of anode and cathode materialsand an ensuing, highly exothermic, chemical reaction; temperatures sohigh as to initiate an extremely vigorous reaction between the sulfur inthe catholyte (in a sodium/sulfur cell, for example) and the aluminumfoil may result. In contrast, breakage of a plug tubesheet (at least ina "two-compartment" cell) would result in exposure of only very limitedamounts of the anode and cathode materials to contact with each other.

Thus, despite the uncertainties posed by the several demonstrated andcontemplated difficulties of making plug tubesheets, a workable methodof fabricating them was still sought.

OBJECTS OF THE INVENTION

The primary object of the invention is to provide a type of tubesheetwhich will permit leak-free hollow fiber battery cells to be morereadily made and scaled up.

An ancillary object is to provide a practical--even automatable--processfor constructing the scaled-up cells.

Another object is to provide a tubesheet of a shape such that it can beformed after the fiber bundle (etc.) has been rolled (or otherwiseformed).

A further object is to provide a tubesheet of a shape such that a largetemperature gradient, from the outer to the inner face of the tubesheet,conducive to attainment of graded porosity in "firing" can beestablished simply by subjecting only the outer end portion of it todirect heating (and allowing the rest of it to be heated by conduction).

An additional object is to provide a tubesheet configuration which canbe more uniformly densified under a time/temperature firing protocolwhich is much less critical and does not require use of a vacuum oven.

A further--and very important--object is to provide a tubesheetconfiguration which can be attained with a slurry which does not include"spheres" or other relatively large, difficult-to-sinter particles.

It is also an object of the present invention to provide a method oftubesheet fabrication in which any broken fibers present in the "green"fiber/tubesheet assembly may be automatically closed off withoutrecourse to post-firing treatments.

Another object is to make it possible to use tubesheet glasses ofessentially the same composition as the fibers.

Yet another object is to provide a tubesheet configuration which permitsthe use of much smaller feed-throughs for effecting seals between thetubesheet and anode and cathode cups.

Still other objects will be made apparent to those knowledgeable in theart by the following specifications and claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 depicts, in vertical perspective, the successive stages in thepreferred mode of fabricating a hollow fiber (etc.)/tubesheet assemblyof the present invention by the method of the present invention.

FIG. 2 depicts, in vertical cross-section, a "two-compartment"sodium/sulfur battery cell in which is incorporated a hollowfiber/tubesheet assembly of the present invention. The portions of thecell which are according to the prior art so identified. Enlarged viewsof the encircled portions of FIG. 2 are given in FIGS. 2A and 2B.

FIG. 3 depicts a "one-compartment" sodium/sulfur battery cell in whichis incorporated a hollow fiber/tubesheet assembly of the presentinvention. The outer casing is seen, in vertical cross-section, afterbeing cut away to expose, intact, the rest of the cell elements to view.The portions of the cell which are at least generally according to theprior art so identified.

SUMMARY OF THE INVENTION

It has been discovered that the foregoing objects can be attained if ahollow fiber (etc.) bundle is first formed with the free ends of thefibers unpotted and:

(1) the rheological and wetting properties of the slurry from which thetubesheet is to be formed and the size range of the particles in theslurry are such that

a. the free fiber ends can be converted to a generally columnar,coherent bundle of coated fiber ends, i.e., to a dipped "brush", by anoperation which comprises dipping them in the slurry,

b. any excess of the slurry over the amount appropriate for the desiredspacing between the fibers (i.e., the desired tubesheet diameter for agiven number of fibers) can be removed from the dipped brush,

and, preferably,

c. the slurry will rise, by capillary action, within the lumens of thefibers (to a greater height in those fibers that may be broken, byreason of their no longer being closed ended);

(2) the free fiber ends are coated with said slurry, any excess of theslurry is removed and the brush of coated fiber ends is allowed orcaused to assume whatever cross-sectional shape is desired for it,

(3) the resulting "green" plug-form tubesheet is allowed todry--optionally, after being recoated as a whole with the same or adifferent but compatible slurry, to provide a more uniform or otherwisemore desirable "skin".

The resulting article has utility in that it may, in a further step, bedried and heated, according to a preselected time/temperature protocoland in a manner such that the tubesheet is converted to an at leastpredominantly ceramified body.

Preferably, nothing is done to prevent entry of the slurry into thefibers and the process includes still another step to ensure that allcompetent fiber ends which have been potted in the slurry are open inthe ceramified tubesheet. That is, a terminal portion of the outer endof the ceramified tubesheet/fiber composite is broken or cut off, thelength of the removed portion being such as to include any tubesheetmaterial which may be present in the lumens of the competent fibers butnot such as to unplug any incompetent fibers. The "fired" assembly willthen be heliumtight without further treatment.

Definitions of Terms

The term "ceramic" is defined for the purposes of this application inaccordance with the broadest meaning given for the term in Webster'sUnabridged Dictionary, 2d edition; i.e., products made from earth (sand,clay, metal oxides, etc.) by the agency of heat, such as glass, enamelsand porcelain, for example.

As used herein, the term "generally columnar" refers to a body for whichthe ratio of its length to its average effective diameter is at least 2or more and applies to elongate bodies which preferably have a regular,geometrical cross-sectional shape--most preferably circular--but mayhave irregularly shaped and/or non-constant cross-sections.

For convenience, the term fiber "bundle" as used herein denotes aplurality of spaced apart fiber lengths, whether "standing" alone or asan element of an assemblage including other elements, such as a cathodefoil or an equivalent current distributing/collecting means, spacingtapes, etc., or whatever.

The term "dipped" as used herein is intended to denote any manner ofcausing the fiber ends to be immersed in a body of the slurry and then,while the fiber ends are vertically oriented, permitting or causing anyportion of that body not taken up by the fiber ends to drain off or tobe otherwise removed. The dipping operation preferably includessubjection of the slurry and/or fiber ends to vibration (to ensure moreuniform emplacement or uptake of the slurry throughout the "brush" offiber ends).

The term "capillary" is used herein as generic to hollow fibers and isused primarily in designating glasses of which hollow fibers arecomposed as "capillary glasses"--to avoid confusion with the establishedmeaning of the term "fiber glass".

As used herein, the term "ceramified" applies to those portions of thefired tubesheet formed either by fusion or sintering of the particlesfrom which they are derived.

The term "at least predominantly ceramified" was used above because itis not necessary for all of the tubesheet particles even to be sintered.The temperature profile along the central axis of the tubesheet during"firing" may be such that the particles subjacent to the "lower" or"inner" surface of the tubesheet remain unsintered; i.e., the lowermostportion of the finished tubesheet may retain the character of a dry but"green" body of the tubesheet particles. At the other extreme, however,the tubesheet particles can all be fused into a unitary, uniformlydensified ceramic mass.

The term "helium tight" refers to a tubesheet in which the fused portionhas an internal structure such that less than 10⁻⁹ c.c. per second ofhelium (measured at standard conditions) can diffuse through thetubesheet or along the fiber/tubesheet interfaces. It should be notedthat the diffusion rate is an absolute rate and is not expressed interms of helium volume diffused (per unit of time) per unit of area; thehelium passes through only by way of leaks, at a rate independent oftubesheet diameter. (When the portions of the fibers depending from thetubesheet are closed-ended, the presence of any cracks, breaks orimperfect end closures may also be detected--as so-called "gross"leaks.)

The term "competent fiber ends" refers to the end portions of fiberswhich are unbroken and are (temporarily or permanently) closed at theirother ends.

It has also been found that strong, not excessively strained,sleeve-type seals between the plug tubesheet and the anode (and cathode)cup feedthroughs can be effected if a good match is made not onlybetween the coefficients of expansion of the seal and ceramifiedtubesheet materials, but also between their glass transitiontemperatures (Tg's).

When embodied as an unfired article, the invention may be defined as:

a bundle of spaced apart ceramic hollow fiber lengths having endportions gathered compactly together and potted in a generally columnarbody of a coherent slurry of powdered ceramic material in avolatilizeable liquid,

the average distance between adjacent said end portions being about 1/2or less of the average distance between the ungathered portions of saidfiber lengths and

said slurry having rheological and wetting properties and the size rangeof the powder particles therein being such that:

(a) said body

(1) could have been formed by an operation comprising dipping said fiberend portions in a quantity of said slurry and restricting the diameterof the resulting "dipped brush",

(2) is convertible by drying and heating to a solid ceramic tubesheetthrough which said end portions of the fibers pass in sealing engagementtherewith and which together with those end portions constitutes acomposite structure.

In a preferred embodiment, the slurry has flowed by capillary attractioninto any of the fiber end portions which were open, to a limited extentwhich is substantially greater for the end portions of any of the fiberswhich are incompetent and the foregoing article has been so converted tosaid composite structure. It is particularly preferred that the fiber"bundle" include a cathode foil, etc., and that the dependent fiber endsbe closed.

A bonus of the plug tubesheet configuration is that a much closer andmuch more uniform spacing between the potted portions of the fibers areattainable than in disc-type tubesheets.

An additional bonus is that a type of leak experienced in disc-typetubesheet/fiber assemblies, i.e, so-called "doublet leaks", can beeliminated in plug tubesheets. That is, when two fibers are notseparated by intervening tubesheet material in a disc-type tubesheet,the result is a leak path along the line of contact between them, fromone face of the tubesheet to the other. The higher firing temperaturewhich can be employed with plug tubesheets results in a sintering actionbetween touching fibers which does effect a seal between them in atleast the zone subjacent to the outer face the tubesheet will have afterthe end of it is removed. This may distort the cross-sections of thefibers involved but not to the extent of seriously restricting theirlumens.

The larger particles in the slurries employed to make the disc-typetubesheets are essential to the slurry rheology required for operabilityof the prior art process for making them. However, elimination of theselarger particles (which are generally spheroidal) is essential to closefiber packing in the plug tubesheet. Fortunately, this omission is morethan just tolerable; it modifies the rheology of the slurry in a wayessential to fabrication of plug form tubesheets. Further, it makespossible "automatic" plugging of the end portions of any incompetentfibers in the bundle; that is, the particles are small enough so thatthe slurry can flow by capillary "attraction" into any open-ended fiberportions during the dipping operation.

It has also been found that it is possible to make assemblies of fibersand plug-form tubesheets utilizing tubesheet materials havingsubstantially higher glass transition temperatures than those whichcould be used in the prior art. This is an unanticipated consequence ofbeing able to form the tubesheet from fine particles, i.e, from slurrieswhich do not include the larger particles present in the prior artslurries.

The present invention is particularly suitable for use in hollow fibertype sodium/sulfur battery cells but is considered suitable for use inhollow fiber type, high temperature battery cells in general. It alsowould appear to offer advantages in the fabrication and operation ofhollow fiber type, high temperature devices other than battery cells.

DETAILED DESCRIPTION Tubesheet Length to Diameter Ratio

As indicated earlier herein, the plug form tubesheets of the presentinvention are generally columnar in shape and have length to averageeffective diameter ratios of at least 2. There is no inherent upperlimit to the ratio but it will be appreciated that the higher the ratiothe more carefully the tubesheet must be handled to avoid breaking it.Further, no advantage is apparent for ratios substantially in excess ofabout 10 to 1 and less efficient use of materials and cell space resultsas the ratio is increased. As a practical matter, ratios within therange of from about 3 to about 6 are highly preferred.

Suitable fiber, tubesheet and sealing materials for the practise of thepresent invention may all be generally categorized as "ceramics" (asdefined earlier herein).

Other properties of the materials selected may vary according to theparticularities of the specific use to be made of the contemplateddevice but in all cases relatively narrow requirements as to thermal andbonding properties must be met.

The tubesheet material must meet an additional requirement. That is, itmust be processable into the form of a slurry which will have anadequately high solids content without being too viscous. As to thermaland bonding properties, the ceramic particles in the slurry must beconvertible, by heating and cooling, to a substantially continuous,ceramic body which is sealingly engaged with the fibers passing throughit and will bond with the sealing glass. The latter body (which may beall of or only an "upper" portion of the tubesheet) must adequatelymatch both the fibers and the formed seal as to coefficients ofexpansion and also must have substantially the same glass transitiontemperature (Tg) as the formed seal. That is, as the seal and tubesheetbodies cool, they must reach their glass transition points atsubstantially the same temperature.

It is to be noted that the internal moisture content of the glass fromwhich the tubesheet is to be made can substantially effect the Tg ofthat glass. Thus, a glass designated as "T-III" (composition given inTable I herein), when dried (by nitrogen purging during melting), hasbeen found to have a 12° higher Tg than when it is not dried. Also, theTg of the tubesheet glass can be altered by loss of any of itscomponents which exert a significant vapor pressure when it is held in amolten state under less than autogenous pressure. For example, T-IIIglass slowly loses HBO₂ under the latter conditions and its Tg increasesaccordingly. Finally, the Tg of the completed tubesheet (or the fusedportion thereof) will not necessarily be the same as that of the glassit was made from. That is, grinding, slurrying, drying and firing(sintering) T-III glass results in a Tg increase of some 15°. Thus, theactual Tg of a tubesheet made from T-III glass could be more than27°higher than the value which might be determined on a specimen of thestarting glass which had not been nitrogen-purged when it was cast andwhich had not been subjected to the tubesheet fabrication procedure.

A candidate sealing glass must possess certain properties in order toqualify for consideration as a cup-to-plug tubesheet seal glass. Thefirst requirements are dictated by the seal environment. For example, ina sodium/sulfur battery cell the glass must have sufficient chemicalresistance toward sodium, sulfur, and sodium polysulfide to maintainseal integrity and must be viscous enough at the 300° C. cell operatingtemperature to prohibit detrimental deformation; the seal glasstransition temperature should be above 300° C. The nature of the sealestablishes further glass property requirements. The seal glass mustform two separate seals: a glass-metal seal with the anode cup and aglass-glass seal with the plug tubesheet. The seal glass must be fluidenough below the melting point of the cup metal (660° C. for aluminum)to be dippable.

The seal glass-to-tubesheet part of the seal places the most stringentproperty requirements of all upon the seal glass. This is discussedlater herein but it may be noted here that glass-to-glass sleeve-typeseals of plug tubesheet size, with unlike glasses, where the seals mustmaintain integrity through a temperature range from room temperature to300° C., are extremely difficult to design and fabricate.

By reason of the effects on the tubesheet glass of the several tubesheetfabrication steps, the Tg of the seal-to-be can be as much as 15° C.lower than the Tg of the tubesheet, even though both are formed from thesame batch of glass.

It has been found that a practical solution to the latter problem is touse a seal glass having a somewhat different composition from thetubesheet glass. In this way, satisfactory matches between both the Tg'sand the linear coefficients of expansion of the seal and tubesheet canbe reproducibly attained.

(It may be noted that a plug tubesheet--including the portions of thefibers potted in it--may readily be made in a size such that it can beutilized per se as a Tg test specimen (the expansion being measuredalong a diameter of the tubesheet).)

It has also been found possible to employ as the tubesheet glass one ofthe same composition as the hollow fibers, when the tubesheet is in theplug form and the slurry it is formed from consists essentially ofparticles with effective radii of curvature substantially less than thatof the fiber lumens. As long as the particles of tubesheet glass makegood contact at points and edges--where localized melting can occurbefore the fibers reach their softening temperatures--an adequatelycontinuous tubesheet structure can be established without closing thefibers. However, the glasses found suitable for capillaries willgenerally not be useable as sealing glasses. They are not fluid enoughbelow the melting point of aluminum to be dippable. A higher meltingtank material could be used but the temperature to which the seal glassmust be heated to effect a conforming seal with the tubesheet is so highthat at least the outermost fibers would be closed.

On the other hand, the Tg of the seal glass must adequately match the Tgof the tubesheet glass; otherwise, cell lifetimes greater than about oneto two weeks are rarely attainable. Thus, it is highly preferable to usea tubesheet (and seal) glass which has a lower Tg than the fiber glass.

When sealing the solder glass to the tubesheet, the temperature israised until the seal glass softens enough to deform to the tubesheet.For example, for the T-III glass, a sealing temperature of 495° C. for15 minutes is required (the sealing temperature is time-dependent). Ifthe Tg of the seal glass is less than that of the tubesheet glass, the(softer) seal glass conforms, as the seal is cooled, to the dimensionalchange of the harder tubesheet glass. Eventually, the seal glass reachesa temperature where it sets up, i.e., will no longer conform to thecooling tubesheet glass without developing internal stresses. This iscalled the seal set point, and occurs at a seal glass temperaturebetween the anneal point and the glass transition temperature. Duringfurther cooling, the seal glass will have a substantially highercoefficient of expansion than the tubesheet glass (which is alreadybelow its Tg). Consequently, the tubesheet/seal composite will bestressed to an undesirable extent. This stress, which cannot be avoidedby resort to slower cooling, will persist under the conditions of usenormally encountered in high temperature battery cells. An intolerabledegree of stressing can only be avoided by an adequate match between theTg's of the seal and tubesheet glasses.

Suitably, the Tg's of the seal and tubesheet glasses do not differ bymore than about 5° C. and their linear coefficients of expansion do notdiffer by more than about 10×10⁻⁷ /° C.

In practice, all glass seals have some internal stress at theiroperating temperature. A good seal is one that functions properly underdesign operating conditions. For most seals, this means the seal stressshould be less than 1500 psi to prevent cracking.

The sealing procedure includes dipping the end of the feed--throughportion of the tank into the molten seal glass, removing it and lettingthe adhered glass "collar" cool. This of course requires that the moltenglass be adequately able to wet the tank material (aluminum, forexample). Since it usually will not be possible to have a close matchbetween the coefficients of expansion of the seal and tank materials, itwill generally be necessary for the portion of the tank in contact withthe seal glass to be relatively thin (5 mils, for example) to avoidintolerable stressing of the glass when the cell is at operatingtemperature. (See FIG. 2B.)

Ceramic materials generally suitable for use as fibers and tubesheets inthe practice of the present invention include those disclosed in theU.S. patents cited previously. Among the latter, the '602, '331, '995and '386 patents are the most informative as to fiber glass compositionsand the '490, '613, '386 and '742 patents are the most informative as totubesheet compositions. For fabrication of high temperature batterycells employing anode and/or cathode materials other than sodium and/orsulfur or of devices other than battery cells, other ceramic materialsmay also be suitable.

Specific compositions are given subsequently herein for variouscombinations of capillary, tubesheet and seal glasses which resulted inhelium leak tight fiber/tubesheet/anode tank assemblies. Lifetime datais also given for complete cells including most of the latterassemblies.

Discussion of compositional effects in more detail is deferred untilafter the following discussion of fabrication procedures.

Fabrication of Fiber/Tubesheet Assemblies

Referring to FIG. 1 of the drawings, in stage A there has been formedand positioned as shown a rolled-up assembly (1) of a central, aluminumtubing mandrel (2), an aluminum foil cathode currentcollector/distributor (3), of which only the outermost wrap is visible,a plurality of parallel holow fibers (4), an aluminum spacer-tape (notshown; see element number 32 in FIGS. 2 and 2A) and two, thin, aluminumpositioning tapes (not shown; see elements number 35 in FIGS. 2 and 2A)coated with a thermally degradeable adhesive. Reference may be had tothe above-cited '868 patent for details of the rolling procedure (exceptfor the use of relatively longer fiber lengths and the omission of thetubesheet material in the assembly at point).

Only the upper end of the mandrel is depicted in stages B through J.

In stage B, the "brush" (5; step A), consisting of the exposed portionsof the fibers (4) (which are closed at their upper ends and open attheir lower ends) has been partially dipped into a slurry (6) in aliquid such as cumene of a tubesheet glass which has been finely groundwith a grinding aid such as hexadecyl amine. The slurry is contained ina cup (7) which has been raised up to accomplish the dipping.

In stage C, cup (6) has been lowered to allow some of the excess slurryto run and drip off of the brush (5), which now includes slurry whichhas been carried into it by a "wicking" action and is coated with asurrounding layer (8) of the slurry.

In stage D, the assembly (1) is rotated (by means not shown) about itsvertical axis while three "shoes" (9) are pressed lightly inwardlyagainst the coated brush and horizontally vibrated (by means describedsubsequently herein) to reduce the effective diameter of the brush,ensure complete and uniform penetration of the brush by the slurry andto facilitate draining off of the remaining excess slurry.

In stage E, the coating (8) has now been substantially reduced inthickness and two encircling ties (10; cotton thread) and (11; finealuminum wire) have been emplaced on the coated brush. The aluminum tiehas been emplaced by looping it around the brush just below the cottontie, then working it down, to squeeze out and push down a little more ofthe slurry, and then tightened gently. (Ordinarily, the aluminum tie isincluded in the portion of the tubesheet removed after "firing".)

In stage F, the coated brush has been rotated about its central axis andaligned, as necessary, by means of a prepositioned means such as one'sfinger or a TEFLON®* cylinder (12; indicated diagramatically), toimprove the concentricity of the fiber layers.

In stage G, the slurry-impregnated and -coated portion (13) of the brush(5) has been allowed to dry until somewhat stiffened and then immersed(dipped) in a slurry (14) of which the composition is the same as ordifferent from that of slurry (6).

In stage H, the slurry has been removed from contact with the assembly,which has then been subjected to several brief periods of rotation tohelp the excess of slurry (14) drain off and to better smooth the newlyformed outer layer (15). If a slurry "tail" (16) is present, it may bereadily cut off, as with scissors.

In stage I, the assembly is being rotated in a forced air flow (from afan) to induce drying (evaporation of the liquid component of theslurry). At this stage, the resulting "green" tubesheet (17) preferablyis baked in vacuo (as described later herein) to complete removal of theliquid slurry medium and to remove as much of the grinding aid aspossible, before stage J is reached.

In stage J, the assembly has been inverted and positioned with the greentubesheet (17) within a metal heating block (18; heat supply andtemperature control means not shown) for sintering of the tubesheet.

The vibration and "squeezing" action of the "shoes" used in stage I ofthe foregoing procedure may be provided by any suitable means. Apreferred such means is that which has been employed in actual practice.That is, an ordinary 3-fingered laboratory clamp is slidably mounted ona vertical rod and has been modified in four respects; the "thumbscrews" have been removed, the spring which normally urges the clamparms apart has been removed, a D.C. relay has been fastened to each armand a grooved plastic block (a "shoe") has been drilled and slipped ontoeach of the three fingers (and secured thereon by means of set screws).

The resultant "vibrator/aligner/squeezer" is not part of the presentinvention and was devised by a co-worker of the present inventors. Theclamp arms, one having a shoe mounted on its single "finger" and theother having a shoe on each of its two fingers, are moved towards oraway from the slurry-impregnated fiber brush by hand. The nominally D.C.relays are caused to "chatter" by applying about 45 volts of A.C. acrosstheir coils (no use is made of the relay "points").

Slurry Composition and Preparation

The composition and preparation of the slurry is generally according tothe disclosure of the above cited U.S. Pat. No. 3,917,490. See also U.S.Pat. Nos. 4,219,613 and 4,403,742, likewise cited above. However, amajor departure is made in that the larger (spherical) glass particlesemployed in the prior art slurries are not included in the slurries usedto make plug tubesheets according to the present invention. This altersthe rheology of the slurry and necessitates modifications in otherprocess parameters.

The liquidity of the slurry is a function of not only the liquid mediumcontent (weight ratio of cumene, for example, to solids) but also therelative amount of the grinding aid (hexadecylamine, for example)employed in production of the powdered tubesheet glass and of thesurface moisture content of the powder particles. It is also a functionof the proportion of Na₂ O in the glass; the T-III glass is relativelylow in Na₂ O and more cumene must be used with it to achieve an adequatedegree of slurry fluidity. This in turn increases a tendency forelongated cracks to form in the cylindrical surface of the ceramifiedtubesheet. Fortunately, however, it is possible to hold down the cumeneto solids ratio, without sacrificing workability of the slurry, byallowing the glass powder to take up a little surface moisture before itis slurried, as by 1 to 2 hours exposure to "dry" room air having arelative humidity of about 3 to 4.5%.

In order to achieve solids contents high enough so that a largeproportion of the tubesheets will be helium leak-tight, it has beenfound desirable to limit mill size during the grinding operation. With a7.5 Kg batch of T-III glass ground in a 1.5 gallon (5.68 liters) mullitemilling jar with 6 Kg of 20×20 mm αAl₂ O₃ cylinders ("balls") themaximum solids/cumene ratio achievable (for appropriate slurry rheology)was 3/1 and only a low proportion of helium-tight tubesheets could bemade. 0.15 Kg batches ground in a 1 quart (0.95 liter) jar with 1.2 Kgof 12×12 mm cylinders were converted to fines with which a 5/1 ratiocould be attained before the slurry viscosity became too high; thehigher ratio resulted in a large proportion of helium-tight tubesheets.

Experiments were carried out with T-III glass ground in the small mill.In these experiments, the effect on helium tightness of the amount ofgrinding aid used and the solids to cumene ratio were assessed. Theresults indicated that the % of grinding aid used (grams per 100 gramsof the glass) could range from about 0.75 to about 1.50 and the solidsto cumene ratio from about 4.0 to about 5.5 to 1, depending on theamount of the grinding aid used. Good results have consistently beenattained by preparing plug tubesheets from T-III glass ground with about1% hexadecylamine ("HDAM") and employed in a solids to cumene ratio offrom about 4.5 to about 5.4 to 1. (Poor results were experienced at asolids to cumene ratio of 5.5 to 1 when only 0.75% of HDAM was used.)

In any case, at least enough of the HDAM should be used to provide amonomolecular layer of it on each of the powder particles. Usually, morewill be desirable.

The relative amount of HDAM required to adequately grind the T-IIIglass, which is relatively soft, is substantially greater than forharder glasses (capillary glass, for example). It is difficult to effecta high degree of removal of HDAM from the soft (borate) glasses and thisapparently is responsible for the bubbles which have been found (by useof a scanning electron microscope) in diamond cut cross-sections offired tubesheets. The bubbles are not connected and do not provide leakpaths, but some expansion can result from their formation (during"bake-out", as discussed later herein).

Another aspect of glass composition is excessive moisture content. Ifthe glass is subject to "weathering" by moisture or if moisture uptakeundesirably alters the performance of the glass as a component of theslurry and/or tubesheet, the glass must be worked with in an environmentof controlled moisture content. Cesia-containing glasses, for example,were found to weather badly in the reduced humidity atmosphere in aso-called "dry room" and had to be worked with in a glove box underdried nitrogen. On the other hand, as discussed above, a certain minimalcontent of moisture in the particles of tubesheet (or seal) glass may beessential to attainment of the desired slurry rheology, at least forsome glasses.

Dry compositions and thermal properties are given in Table I for anumber of representative glasses which have been used as fibers,tubesheets and seals in fiber/tubesheet assemblies and cells.

The most preferred glasses for use as seal and/or tubesheet glasses arethe ternary glasses, T-I, II, III and IV listed in the Table. See alsoExample 6 herein and the discussion following it.

The tubesheet and seal preferably consist essentially of Na₂ /B₂ O₃/SiO₂ glasses in which the B₂ O₃ to Na₂ O mole ratios are within therange from about 9 to about 24 to 1. It is particularly preferred thatthe B₂ O₃ to SiO₂ mole ratio in the tubesheet glass be about equal tothe B₂ O₃ to SiO₂ ratio in the seal glass.

                                      TABLE I                                     __________________________________________________________________________    CAPILLARY, TUBESHEET AND SEAL GLASSES                                                                                 Thermal                                                                 Nominal.sup.1                                                                       Coef. of                              Wt. % Composition/Mole Ratio      Glass Expansion                             Glass                      NaF    Trans.                                                                              in Linear                             Designation                                                                           Na.sub.2 O                                                                        Cs.sub.2 O                                                                       B.sub.2 O.sub.3                                                                  SiO.sub.2                                                                          NaCl                                                                              (or)   Temp. Range: (α)                                                                      Used as                       __________________________________________________________________________    Hard    3.1 42.0                                                                             53.1                                                                             1.9                                                         Cesia-1                --  --     380° C.                                                                      116 × 10.sup.-7 /°C.                                                     Tubesheet                             1.0  3.0                                                                             15.2                                                                             0.6                                                         Hard    1.0  2.9                                                                             14.3                                                                             0.6                           Tubesheet                     Cesia-2 2.6 35.4                                                                             59.7                                                                             2.2                                                         Soft Cesia             --  --     355   116     Tubesheet                             1.0  3.0                                                                             20.4                                                                             0.9                                                                 27.85  62.56                                                                            5.40 4.20                     Tubesheet &                   D-406       --             --     480   116                                           1.0    2.00                                                                             0.20 0.16                     Capillaries                           3.5    93.7                                                                             2.8                           Tubesheet &                   T-I         --         --  --     285   106                                           1.0    23.8                                                                             0.8                           Seal                                  4.5    92.7                                                                             2.7                                                         T-II        --         --  --     305   105     Seal                                  1.0    18.2                                                                             0.6                                                                 6.5    90.8                                                                             2.7                           Tubesheet &                   T-III       --         --  --     328   103                                           1.0    12.4                                                                             0.4                           Seal                                  8.8    88.6                                                                             2.6                           Tubesheet &                   T-IV        --         --  --     342   102                                           1.0    9.0                                                                              0.3                           Seal                                  2.4    89.9                                                                             2.3      5.4                                                AC1         --         --         --    --      Seal                                  1.0    33.4                                                                             1.0      3.3                                                        3.4    91.2                                                                             2.7      2.8                                                AC2         --         --         --    --      Seal                                  1.0    23.9                                                                             0.8      1.2                                                CZN     1.0 -- 0.6                                                                              0.5  --  0.6                                                                              Al.sub.2 O.sub.3                                                                  450   135     Capillaries                   DKY     1.0 -- 0.9                                                                              1.0  --  0.3                                                                              Al.sub.2 O.sub.3                                                                  465   125     Capillaries                   __________________________________________________________________________     Note:                                                                         .sup.1 As determined on cast and annealed sample of the glass. Not the        same as the Tg of a finished tubesheet, seal or capillary.               

Bake-out and Sintering Operations

Removal (decomposition) of the glue on the spacing and positioning tapesin the fiber bundle requires higher temperatures than does removal ofthe hexadecyl (or other such) amine from the tubesheet but sintering canbe accomplished at temperatures below those required for the glueoxidation. Accordingly, either of two different bake out/sinteringprotocols may be employed.

In both protocols, sintering is done in an aluminum heating block whileexposed to room air (ambient temperature 20°-24° C., relative humidity3-4.5%) at atmospheric pressure.

In the less preferred protocol, the tubesheet is first dried by heatingit, at a rate of 300° C./hour, to 100°±5° C. and holding it attemperature for 0.5 hour. It is then largely freed of the amine byheating it, at a rate of 600° C./hour, to 350°±3° C. and holdingtemperature for 0.75 hour. The actual sintering is accomplished by nextheating the tubesheet, at a rate of 400° C./hour, to 470°±1° C. and"soaked" at temperature for 1 hour. Cooling follows, at a rate 300°C./hour or less. The fiber/foil bundle is then placed in a heater blockin vacuo and heated to a temperature within the range of from about 240°to about 260° C. and held there for about 2 hours.

In the more preferred protocol both amine removal and glue decompositionare achieved prior to sintering. The dried tubesheet/fiber (etc.) bundleassembly is heated as a whole, and in vacuo, to a temperature within therange of from about 240° to about 260° C. and held in that range forabout 2 hours. The assembly is then cooled and the tubesheet heated at350° and sintered at 470° C., as in the preceding protocol.

It has been found that "foaming", apparently due to formation of bubblesof residual amine, occurs during sintering (and/or post-sinteringheating steps such as bake-out or sealing) in tubesheets densifiedaccording to the first protocol. This can result in as much as a 9%expansion in the tubesheet volume. However, tubesheets densified in themanner of the second protocol undergo contraction, rather thanexpansion. This contraction may be sufficient to cause longitudinaltension cracking in the "fired" tubesheet surface when the cumenecontent of the slurry is relatively high, but otherwise is preferable.

Both protocols are advantageous in comparison to that employed fordisc-type tubesheets, in which the use of a vacuum oven with very closetemperature control is required.

To facilitate understanding of how the sintered tubesheet isincorporated in the cell, reference is now made to FIG. 2 of thedrawings, in which a "two compartment", plug tubesheet cell (sanselectrochemical reactants) is depicted. The portions of the cell whichare according to the prior art are so identified in the Figure (althoughthe cell in its entirety is novel as a combination of old and newelements. The cell (indicated generally by the number 19) comprises analuminum anode (or anolyte) tank (20), a cathode (or catholyte) tank(21), a plug tubesheet (22) with which tanks (20) and (21) are sealinglyengaged by a body (23) of seal glass. Tanks (20) and (21) includepreformed fill spouts ("ports") (24 and 25, respectively) which are openbut would be closed, as indicated in phantom, after the tanks have beenfilled with the anolyte and catholyte. Tank (21) contains an innerassembly (indicated generally by the number 26) which includes a largenumber of hollow glass fibers (27), of which only a few are shown. Thefibers have upper end-portions (28)--which extend through and are"potted" in the tubesheet (22)--and have lower portions (29) which areclosed-ended. The assembly (26) also includes a central, aluminummandrel (30) about which it was rolled up, wraps (31) of a foil-form,aluminum cathodic current collector/distributor, wraps (32) of analuminum spacing tape--the thickness of which is greater than thediameter of the fibers. Several, radially disposed heli-arc weld beads(33; only one shown) are formed on the bottom of the roll to ensure goodelectrical contact between the foil and tape wraps and an aluminum tail(34) is joined to the roll bottom by one of weld beads (33). The tailwill be straightened and joined at its lower end in the body of metalformed when the spout (25) is closed--as by crimping and/or welding.(Spouts (24) and (25) will also function as electrical terminal posts inthe complete cell.) The inner assembly further includes wraps (35) oftwo thin aluminum tapes to which the fibers were adhered by a layer ofadhesive when the roll was formed, in order to maintain a preselecteddistance between adjacent fibers. (The adhesive has been thermallydecomposed and removed.)

FIG. 2A is a magnified depiction of the encircled portion of the innerassembly (26). It will be seen that the wraps (31) of the currentcollecting/distributing foil are pierced by openings (36) to facilitatecatholyte introduction between adjacent fibers and between successivefiber/foil wraps.

FIG. 2B is a magnified depiction of the encircled portion of thetubesheet to anode tank seal. The lower end of the anode tank (20) is acollar-shaped "feedthrough" (indicated generally by the number 37),which includes a lowermost section (38) which has been machined to areduced thickness (5 mils, for example) and pre-engaged with the glassfrom which seal (23) was later formed (as described subsequentlyherein).

The bulge (39) at the lower end of the tubesheet in FIG. 2 resuts fromthe presence of the cotton tie (40) which was introduced (as element 10)in stage E, FIG. 1.

Referring to FIG. 3, the depicted "one compartment" cell (indicatedgenerally by the number 41) differs from the two-compartment cell ofFIG. 2 in several respects. The cell comprises an inner assembly (42)consisting of a foil-wrapped fiber bundle (43), a plug tubesheet (44), aglass seal (45) and an anolyte tank (46) which is extended by an anolytefill spout (47) which also functions as the negative terminal of thecell. The inner assembly is disposed, together with a cylindrical,stainless steel spacer (48), in a stainless steel casing (49) whichincludes a catholyte fill spout (50) and is sealingly engaged by a weldbead (51) with the bottom rim of a lower, cylindrical, metalic portion(52) of a pre-fabricated, insulator/seal (indicated generally by number53). The upper rim of element (52) is sealingly engaged with an annular,hard glass insulator (54) which also is sealingly engaged with the lowerrim of a bell-shaped, metallic extension (55). The upper end of thelatter "bell" is sealingly engaged with the periphery of spout (47) byan encircling weld bead (56). The metal portions (52) and (55) of theinsulator/seal (53) consist of KOVAR, a well-known alloy of iron, nickeland cobalt. Insulating seals of the latter type are made and sold byLarson Electronic Glass Co., Redwood City, California, U.S.A. Anelectrical connection between the bottom of element (43) and the casing(49) is provided by a limp length (57) of a heavy gauge aluminum foilthrough end welds (not shown).

The lower portion (not separately numbered) of casing (49) functions athe catholyte container.

Tubesheet-to-Tank Seals

The seal between the tubesheet and the anode tank is made by: (1)dipping the machined portion of the tank feed-through (see FIG. 2B ofthe drawings) in the molten seal glass, (2) withdrawing it whileexerting sufficient gas pressure inside the tank so that the adheredglass will depend from the feed-through in the form of an open-endedcylinder or collar, (3) allowing the collar to solidify, (4) insertingthe plug tubesheet within the collar, (5) heating the collar (andtubesheet) until the collar melts and contracts annularly against thetubesheet surface, and (6) allowing the assembly to cool slowly.

If seals are to be effected to both the anode and cathode tanks (as inFIG. 2 of the drawings), the procedure is generally the same. However,the foil wrapped fiber bundle and the tubesheet must be positioned inthe (open bottomed) cathode tank and pre-glassed feed-through,respectively, before the anode tank feed-through is slipped down aroundthe then protruding tubesheet. Also, the bottom of the cathode tankpreferably is not welded on until after both seals have been effected(simultaneously). As shown in FIG. 2, the sealing glass may constitute asingle body of glass when the sealing operation has been completed.

In greater detail, the preceding step (5) is carried out by resort toeither induction or simple resistive/conductive heating--the latterbeing considerably preferred.

In the inductive heating method, the tubesheet and bundle assembly andthe dipped anode tank feed-through are positioned within a pyrex tube,together with a carbon ring, having the form of a short cylinder, and aVYCOR supporting/spacing tube. The carbon ring rests on the latter tubeand surrounds the dipped feed-through. The entire tube and its contentsare preheated in a heater block to about 400° C., under an inertatmosphere, and then lowered through an induction coil until the carbonring is within the coil. The coil is energized, thereby heating the ringto a red glow, the ring in turn heating the seal glass by irradiation.

In the preferred heating method (as applied in fabrication of aone-compartment cell), the entire anode tank is positioned between thetwo halves of a "split" nickel sleeve which is shaped to support thetank without hindering insertion of the tubesheet in the dippedfeedthrough. The nickel cylinder is slid into a closely fitting,thick-walled, copper cylinder around which an electrical heating bandand an outer layer of thermal insulation are disposed. Thetubesheet/fiber (etc.) assembly is positioned by a centering jig andraised by a height-adjustable means, such as a laboratory jack, untilthe tubesheet is properly positioned within the glass-dippedfeedthrough. The heating band is turned on and the seal glass collarbrought to a preselected temperature and held there for a preselectedperiod--which is temperature dependent. (For T-III seal glass, 15minutes at 495° C. has been found suitable.) The heat is then turned offand the entire apparatus allowed to cool slowly. The split sleeve andthe anode tank/tubesheet/fiber (etc.) assembly are lifted out andseparated.

EXAMPLES

The following examples are for purposes of illustration and are not tobe construed as limiting the present invention in a manner inconsistentwith the claims in this patent.

EXAMPLE 1

A slurry of 4.8 grams of cumene and 30 grams of powdered "hard cesia-1glass" (see Table I), which had been ground with hexadecylamine in themanner of U.S. Pat. No. 3,917,490, was made up, vacuum deaerated,bottled and placed in an "ultrasonic bath". 3800 Hollow glass fibers(D-406 glass, Table I), each having a length of 6 inches (15.24 cm), anO.D. of 80 microns, an I.D. of 50 microns and being closed at one end,were laid down in ladder fashion on properly spaced, "spacing" and"positioning" aluminum foil tapes (FIG. 2) coated with rubber cement (bysolvent evaporation). The number of fibers per cm of the ladder lengthwas 20. A 1.9 meters long section of the ladder was rolled up with thecoated tapes and a 4.5" (11.43 cm) wide, 12.7 microns (5 mil) thick,air-baked, molybdenum-coated aluminum foil strip about 2 meters long.The thickness of the spacing tape was such that the successive wraps ofthe latter foil were 127 microns (5.0 mils) apart. The open-ended"bending section" of the resultant fiber bundle, 2.5" (6.35 cm) long,was slowly (5 minutes) lowered into the slurry--which was still beingsubjected to ultrasonic vibration--to a depth of about 1.25" (3.18 cm)and held there for about 60 seconds. The resulting "dipped brush" wasthen withdrawn and "milked" of excess slurry. One cotton and onealuminum tie were applied (see FIG. 1, stage E) and the constrictedbundle end redipped in the slurry. The resulting nascent tubesheet wasslowly dried and then inserted to a depth of 1" (2.54 cm) in a 1/2"diameter (surface darkened) well of an aluminum heating block, withouttouching the wall. The block was then heated as follows: 0.5 hour at100° C., 0.5 hour at 400° C. and then 1 hour at 505° C. After beingallowed to cool slowly, the "fired" tubesheet was shortened (to ensurethat all fibers were open) using a pair of side-cutting pliers to snapoff a 0.25" (0.64 cm) end piece (including the aluminum tie). Theresulting tubesheet/fibers (etc.) assembly was tested and found to behelium-tight (as defined earlier herein).

EXAMPLE 2

A smaller (500 fiber) plug tubesheet/fiber (etc.) assembly made in themanner of Example 1 but using a "soft cesia" glass (Table I) as thetubesheet material was made, found to be helium-tight and incorporatedin a complete, one-compartment cell, using the T-I "ternary" glass forthe anode tank to tubesheet seal. The cell was charged with sodium andsulfur and was satisfactorily charge/discharge cycled for 8 weeks (atabout 300° C.).

EXAMPLE 3

Before the niceties of matching the Tg's of the tubesheet and seal wereappreciated, fifteen helium-tight, two-compartment, plug tubesheet cellswere fabricated with tubesheets made of capillary glass (D-406) andinduction-heated seals made of either T-I or AC1 glass. Of the eightcells in which the seals were made of T-I glass, two lasted 6 days whencharged with sodium and sulfur and cycled at ˜300° C. a total of 14times; none of the other five lasted more than two days. Of the sevencells in which the seals were made with AC1 glass, two lasted 4 days(10, 11 cycles) and the rest lasted 3 days or less.

EXAMPLE 4

Some improvement over the results in Example 3 was experienced when anumber of one-compartment cells were made using D-406 fibers, D-406tubesheets and T-III seals, but only a few cells lasted longer than 6days, i.e., up to 13 days (11 cycles), even though all cells werehelium-tight when put on test.

Two one-compartment cells having nominal capacities of 12 ampere hourswere fabricated with D-406 fibers, T-III tubesheets and T-IV seals. Onelasted only 8 days (10 cycles) but the other lasted 31 days (42 cycles).(In both cells, the molybdenum-coated aluminum cathode foil had beenair-baked to form a surface layer of molybdenum oxides.)

EXAMPLE 5

A total of 26 one-compartment, helium-tight, plug tubesheet cells (12ampere hours nominal capacity) were fabricated. In each of these cells,the fibers consisted of D-406 glass and the tubesheet and seal were madeof the same ternary glass (T-III). In eight of these cells, themolybdenum-coated, aluminum cathode foils had not been air-baked. In theother 18 cells, the foil had been air-baked (air-baking is the subjectof a separate patent application, Ser. No. 494,595 filed May 13, 1983and now allowed). The tubesheet glass used in half of the latter 18cells had been nitrogen purged. This was not true of the glass used inany of the other cells in either group. The lifetimes of these 26 cellsat 300° C. are given in the following Table II.

The variation in lifetime shown by nominally identical cells undernominally identical conditions is illustrative of the inherentdifficulties of reproducibly fabricating hollow fiber cells by currenttechniques. However, the benefits of using closely matching seal andtubesheet glasses and the mismatching resulting from nitrogen purging ofonly one of the supposedly matched glasses is nevertheless apparent fromthe data in the Table.

                  TABLE II                                                        ______________________________________                                        LIFETIMES OF CELLS                                                            IN WHICH T-III GLASS USED IN                                                  BOTH TUBESHEET AND SEAL                                                       Cell  Foil Air Lifetime                                                       No.   Baked?   Days   Cycles                                                                              Comments                                          ______________________________________                                        PA 4  No       31     38                                                      PA 5  "         4     10    Low sodium found.                                 PA 6  "        12     9     Open circuit after cycle 8.                                                   Low Na.                                           PA 9  "        37     8     Open circuit after cycle 8.                                                   Low Na.                                           PA 10 "        21     20                                                      PA 20 "        48     39                                                      PA 29 "        78     44                                                      PA 30 "        12     14                                                      PA 47 Yes      159    214                                                     PA 48 "        23     "                                                       PA 51 "        13     12                                                      PA 53 "        18     20                                                      PA 113                                                                              "        128    189   Low Na. Cycled at 5 amp. hr.                      PA 182                                                                              "        100    126                                                     PA 183                                                                              "         3     4                                                       PA 185                                                                              "        125    116   Cell heater failed.                               PA 187                                                                              "         1     2                                                       PA 118                                                                              "         3     3     Tubesheet glass N.sub.2 -purged.                  PA 120                                                                              "        40     28       "                                              PA 129                                                                              "         1     1       "                                               PA 130                                                                              "         0     0       "                                               PA 132                                                                              "         4     2       "                                               PA 133                                                                              "         5     4       "                                               PA 137                                                                              "         1     1       "                                               PA 138                                                                              "         1     1       "                                               PA 142                                                                              "         1     1       "                                               ______________________________________                                    

EXAMPLE 6

One 6 ampere hour and two 12 ampere hour one-compartment cells whichwere relatively long lived were fabricated with D-406 fibers and withthe tubesheet and seal glasses given in Table III following:

                  TABLE III                                                       ______________________________________                                        LONGER LIVED CELLS                                                            WITH "TERNARY" TUBESHEET & SEAL GLASSES                                       Amp. Hr.      GLASSES       LIFETIMES                                         Cell No.                                                                              Rating    Tubesheet Seal  Days  Cycles                                ______________________________________                                        PA 279   6        T-I       T-II  196   252                                   PA 214  12        T-III     T-IV  205   107                                   PA 34   12        T-I       T-III 252   116                                   ______________________________________                                    

Table IV, following, includes the Tg and α values and the compositions(from Table I) for the tubesheet and seal glasses used in cells PA 47,PA 113 and PA 185 (Table II) and in cells PA 279, PA 214 and PB 34(Table III). Also given in Table IV are the B₂ O₃ /SiO₂ mole ratios foreach tubesheet and seal glass. For each cell, the ratio of the latterratios is also shown.

                                      TABLE IV                                    __________________________________________________________________________    GLASS MATCHES IN LONGER LIVED PLUG TUBESHEET CELLS                                                         Ratio                                            Cell                                                                              Tubesheet Glass                                                                           Seal Glass   of  Lifetime                                     No. B.sub.2 O.sub.3 /SiO.sub.2                                                          Tg.sup.1                                                                         α.sup.1                                                                    B.sub.2 O.sub.3 /SiO.sub.2                                                           Tg.sup.1                                                                         α.sup.1                                                                    Ratios                                                                            in Days                                      __________________________________________________________________________    PA 47                                                                             12.4/0.4 =                                                                          328                                                                              103.sup.2                                                                        Same as for tubesheet                                                                      1   159                                              31                                                                        PA 113                                                                            12.4/0.4 =                                                                          "  "  "            1   128                                              31                                                                        PA 185                                                                            12.4/0.4 =                                                                          "  "  "            1   125                                              31                                                                        PA 279                                                                            23.8/0.8 =                                                                          285                                                                              106                                                                              18.2/0.6 =                                                                           305                                                                              105                                                                              1.0.sup.3                                                                         196                                              30*         30                                                            PA 214                                                                            12.4/0.4 =                                                                          328                                                                              103                                                                              9.0/0.3 =                                                                            342                                                                              102                                                                              1.0 205                                              31          30                                                            PB 34                                                                             23.8/0.8 =                                                                          285                                                                              106                                                                              12.44/0.43 =                                                                         328                                                                              103                                                                              1.0 252                                              30          31                                                            __________________________________________________________________________     NOTES:                                                                        .sup.1 Nominal for glass, not necessarily same for tubesheet or seal per      se.                                                                           .sup.2 × 10.sup.-7 /°C.                                          .sup.3 29.8/30.3 [ = 0.98] = 1.0                                              *23.8/0.8 [ = 29.8] = 30.                                                

It is apparent from the foregoing examples than longer cell lives can beachieved when the compositions of the tubesheet and seal glasses matchclosely. For cells in which both the tubesheet and seal glasses areternary (sodium borosilicate glasses, the longest cell lives wouldappear to be achievable when the B₂ O₃ to Na₂ O mole ratios are withinthe range of from about 9 to about 24 (and the B₂ O₃ /SiO₂ ratio isabout the same in both the tubesheet and seal glass): i.e., preferablyfrom about 30 to about 31.

Variations Within the Ambit of the Invention

It is highly preferred, but may not be essential, to form the ceramicpowder particles in the presence of a grinding aid (such ashexadecylamine, for example). That is, the amine or equivalent compoundcan be combined with the powder, in any other suitable fashion, after ithas been formed (as by grinding the glass without a grinding aid andsieving out the smaller particles for use as the powder, for example).The powder may, for example, be stirred with a solution of the amine ina highly volatile solvent, filtered out and dried.

It is highly preferred, but not essential, that the potted fiber ends inthe tubesheet be uniformly spaced apart (or uniformly "packed"). Thespacing between adjacent fiber ends can vary not only within any givencross-section but between different cross-sections along the axis of thecolumn.

Again, it is highly preferred, but not essential, that the vertical axisof the tubesheet be coextensive with the vertical axis of the fiber,foil, etc., bundle; the tubesheet axis may be radially offset, eventhough this results in a varying degree of curvature in the fiber endspotted in the tubesheet.

Similarly, it is preferred that the axis of the tubesheet, if notcoextensive with the axis of the fiber bundle, be at least parallelthereto. However, this is not essential; the tubesheet axis may beinclined with respect to the bundle axis. PG,46

It is highly preferred, but not essential, that the portions of thefiber lengths constituting the unpotted part of the "bundle" be disposedin parallel array. The fibers may be disposed in any otherwise suitablearrangement within the bundle from which free end portions may extend insuch manner as to be gatherable into a "brush" which may be potted in aplug-shaped tubesheet in essentially the manner disclosed herein.

It is to be noted that, for the purposes of the claims appended to thesespecifications, a plurality of fiber bundles having end portions pottedin a common tubesheet are to be considered as portions of a single"bundle". This option includes the case in which the fiber bundle isgenerally U-shaped and both ends of the fibers are open and terminate inthe same (outer) surface of a common, plug-form tubesheet.

In another alternative, the fiber (lengths) may be either rectilinear,arched or generally U-shaped and pass at opposite ends through twoseparate plug-form tubesheets. In this case, both ends of the fiberswill usually be open and the two tubesheets may or may not be formedsimultaneously.

It will be appreciated that whenever both ends of the fibers are open,some expedient--such as plugging one end of each fiber length with asubsequently removeable wax, for example--should be resorted to in orderto ensure that the slurry does not deeply enter unbroken fiber lengthsduring the first potting (dipping, etc., or whatever) operation, i.e.,potting of the other ends of the fibers. The temporary plugs are thenremoved from the unpotted ends before proceeding to pot them.

It also is to be noted that the portions of the fibers which are to be"dipped" in the slurry do not have to be open. For example, closed endsmay be dipped and then opened after "firing", by cutting off a terminalportion of the composite tubesheet/fiber structure. (This of course willnecessitate post-firing treatment by one of several known methods toplug off any incompetent fibers.)

It should be noted that the shape of the bottom portion of the anodetank (or of the top portion of cathode tank) between the tank wall andthe feedthrough which conforms to the tubesheet is not critical,although the conical shapes shown in FIGS. 2 and 3 are preferred.

Note

It is to be understood that in all embodiments of the present invention,the ratio of the average effective diameter of the unpotted portion ofthe fiber length bundle to the average effective diameter of the pottedportion will be such that the ratio of average spacing between adjacentlengths in the potted and unpotted portions will be as specified; i.e.,equal to or less than about 1/2.

What is claimed is:
 1. A bundle of spaced apart, ceramic, hollow fiberlengths having end portions gathered compactly together and potted in agenerally columnar body of a coherent slurry of a powdered ceramicmaterial in a volatilizeable liquid,the unpotted portions of said fiberlengths defining a larger diameter part of said bundle, the averagedistance adjacent said potted portions being about 1/2 or less of theaverage distance between adjacent said fiber portions defining saidlarger diameter part of said bundle, and said slurry being convertible,by drying, heating and cooling, to a solid ceramic tubesheet throughwhich said potted portions pass in sealing engagement therewith andwhich together with those potted portions contitutes a compositestructure.
 2. A bundle as in claim 1 wherein only one end portion ofeach fiber length has been potted, and the other end portion terminatesin a closure formed by fusing the fiber material.
 3. A bundle as inclaim 2 wherein said powdered ceramic material consists essentially ofNa₂ O, B₂ O₃ and SiO₂ in a mol ratio within the range of from about1.0/9.0/0.3 to about 1.0/24.0/0.8.
 4. A bundle as in claim 3 wherein thesolids to liquid weight ratio in said slurry is within the range of fromabout 4.5 to about 5.4 to 1, said liquid is cumene and said slurrycontains an amount of hexadecylamine at least sufficient to provide amonomolecular layer thereof on the particles constituting said powder.5. A bundle as in claim 4 wherein the weight of said amine in the slurryis about 1% of the weight of said powder therein.
 6. A bundle as inclaim 1 wherein said body has been so converted.
 7. A bundle as in claim2 wherein the potted portions of the fibers were open-ended, the slurryhas flowed into them to a limited extent which is substantially greaterfor any end portions of said fiber lengths which are incompetent; saidbody has been so converted and a terminal portion of the resutingcomposite structure has been removed, the length of the removed portionbeing such as to include the tubesheet material in the lumens of thecompetent fibers but not such as to unplug any incompetent fibers.
 8. Abundle as in claim 3 wherein the potted portions of the fibers wereopen-ended, the slurry has fowed into them to a limited extent which issubstantially greater for any end portions of said fiber lengths whichare incompetent; said body has been so converted and a terminal portionof the resulting composite structure has been removed, the length of theremoved portion being such as to include the tubesheet material in thelumens of the competent fibers but not such as to unplug any incompetentfibers.
 9. A bundle as in Caim 4 wherein the potted portions of thefibers were open-ended, the slurry has flowed into them to a limitedextent which is substantialy greater for any end portions of said fiberlengths which are incompetent; said body has been so converted and aterminal portion of the resulting composite structure has been removed,the length of the removed portion being such as to include the tubesheetmaterial in the lumens of the competent fibers but not such as to unplugany incompetent fibers.
 10. A bundle as in claim 9 wherein said fiberlengths have been formed from a glass consisting esstentially of Na₂ O,B₂ O₃, SiO₂ and NaCl in a mole ratio of about 1.0/2.0/0.2/0.16,respectively.
 11. A bundle as in claim 2 wherein said fiber lengths andsaid tubesheet consist essentially of the same ceramic material.
 12. Thebundle of claim 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or 11 wherein said bodyhas been formed by dipping said fiber end portions in a quantity of saidslurry to form a dipped brush of them and then restricting the diameterof said brush.